1. Field of the Invention
The present invention relates to a cell selection scheme in a CDMA (Code Division Multiple Access) mobile communication system and a base station device and a mobile station device suitable for a cell selection scheme.
2. Description of the Background Art
In the CDMA mobile communication scheme, a service area is divided into a plurality of unit regions called cells and one base station is provided in each cell so that a mobile station located in one cell within the service area carries out communications with the base station provided in that cell through radio channels.
Each base station is constantly transmitting a pilot channel, and the individual pilot channel is identifiable in some way when the pilot channel is received at the mobile station. For example, in the FDMA system, different frequencies are used for different pilot channels. In the CDMA system, the individual pilot channel becomes identifiable for the mobile station by using different spread codes for different pilot channels.
The mobile station measures receiving levels of a plurality of pilot channels, and makes the cell selection according to the measured values. More specifically, the mobile station despreads the received signals using a plurality of spread codes that can be used for the pilot channels, measures the receiving levels, and stores the measured values and the corresponding spread codes. After the measurement for all the spread codes is finished, the despreading and receiving processing is carried out for a pilot channel corresponding to the spread code for which the receiving level is largest, and the validity of the communication is judged by checking whether that pilot channel can be decoded without an error, whether the communication within that cell is permitted or not according to the decoded information, etc. When it is judged as valid, this cell is set as a located cell. If it is not valid, the receiving processing for a next pilot channel in an order of the receiving levels is carried out and the validity of the communication is judged similarly, until a valid cell is set as a located cell.
A time required for the pilot channel receiving level measurement processing in the above described cell selection processing is as much as multiple times of the spread code period per one pilot channel measurement, for the following reasons.
As an illustrative example, a case of using a matched filter for the pilot channel receiving level measurement processing will be described. FIG. 1 shows an exemplary output of the matched filter when a signal spread by a spread code is entered. As shown in FIG. 1, a peak signal is outputted at an interval of one spread code period, and an output level of this peak signal is taken as the receiving level. In the actual receiving level measurement processing, in order to improve the measurement accuracy, the output levels are measured over several periods at a timing of the peak signal detected in the first spread code period and an average value of these measured output levels is taken as the measured value of the receiving level. Consequently, the receiving level measurement for one spread code is going to require as much time as multiple times of the spread code period.
The above description is an exemplary case of using the matched filter, but the matched filter has a serious drawback in that the hardware size is relatively large and the power consumption is large, so that a sliding correlator which is more advantageous in terms of the hardware size and the power consumption is often used in practice. However, in a case of using the sliding correlator, the processing time is as much as the processing time of the matched filter multiplied by the spread code period, so that even longer processing time is going to be required.
In addition, this much of the processing time is required as many times as a number of spread codes that can be used as the pilot channels, so that the cell selection processing is going to be require a rather long time. This implies that a long time will be required after the power of the mobile station is turned on until the mobile station becomes operable, for instance.
As a scheme for resolving this problem, there is a scheme in which the pilot channels are made identifiable by assigning mutually different spread code phases to different pilot channels. FIG. 2 shows an exemplary output of the matched filter used for the pilot channel receiving level measurement processing in a case of using an identical spread code and different spread code phases for different pilot channels. In this example, the spread code phases of three pilot channels are sequentially displaced by 1/3 of the spread code period. As shown in FIG. 2, peaks corresponding to these three pilot channels appear at an interval of the spread code phase difference between two pilot channels, and a peak corresponding to one pilot channel appears at an interval of the spread code period similarly as in a case of FIG. 1.
In this scheme, peaks corresponding to a plurality of pilot channels can be detected by the measurement for a single spread code, so that the receiving levels of a plurality of pilot channels can be obtained by measuring each of these peaks. In this manner, by assigning N sets of mutually different spread code phases for a single spread code with respect to N sets of pilot channels, the receiving level measurement can be carried out in a time as short as 1/N of that required in a case of identifying the pilot channels using mutually different spread codes.
Here, in order to use these mutually different spread code phases assigned to a plurality of pilot channels, it is necessary for all of the base stations to share a common reference timing, so that a spread code phase difference for each pilot channel can be set up with respect to this reference timing. This is because, if the pilot channel is transmitted at an arbitrary spread code phase by each base station, there is a possibility for a plurality of pilot channels to be transmitted at nearly identical spread code phases. In such a case, the mobile station would not be able to identify these plurality of pilot channels which are transmitted at nearly identical spread code phases, so that it would becomes impossible to carry out the cell selection using these pilot channels.
FIG. 3 shows a timing chart in a case of assigning mutually different spread code phase differences with respect to the reference timing to three pilot channels. As shown in FIG. 3, by using the common reference timing for all the base stations and assigning the spread code phase difference with respect to this reference timing to each the pilot channel, it is possible to realize the mutually different spread code phases for different pilot channels.
However, in order to share the common reference timing among a plurality of base stations, it is necessary to provide a very complicated timing synchronization means. Up to the present, a scheme for carrying out radio or wire communications among the base stations to measure the round trip delay and a scheme for utilizing GPS (Global Positioning System) have been proposed and practiced in some cases, but there has been an associated difficulty concerning the complicated hardware configuration.
In particular, in conjunction with a future development of the mobile communication service, it is expected that greater degrees of freedom in the base station installment as well as a smaller size of the base station will be demanded. Consequently, it is not feasible to provide a function for sharing the common reference timing in every base station from a point of view of the required hardware configuration, and it is expected that this function can be provided in only a limited number of base stations.
Thus, in the scheme using different spread codes for different pilot channels, there has been a problem of a long processing time required for the cell selection processing. In contrast, in the scheme using different spread code phases for different pilot channels, the processing time required for the cell selection processing can be shortened but there is a need for all the base stations to share the common reference timing, and there has been a problem that a complicated hardware configuration required in providing this function at every base station makes this scheme difficult to practice.
On the other hand, in the mobile communication system, one cell is often subdivided into a plurality of sectors in order to increase the radio channel capacity using an increased number of sectors per one cell. For example, FIG. 4 shows a cell C1 which is subdivided into three sectors: the first sector S1, the second sector S2 and the third sector S3.
In a case of using such sectors, there is a need for a located sector judgement processing in order to judge a sector in which the mobile station is currently located. To this end, it is necessary to provide different pilot channels for different sectors.
However, when different pilot channels are provided for different sectors and different spread codes are assigned to different pilot channels in order to carry out the located sector judgement processing, there arises a problem that a required number of spread codes for the pilot channels in the system increases as a number of sectors per one cell increases.
The mobile station realizes the activation at a time of power on by sequentially carrying out the receiving level measurement processing for the pilot channels corresponding to all the spread codes stored in the mobile station, and when a number of spread codes for the pilot channels increases in conjunction with the use of sectors as described above, it becomes necessary for the mobile station to carry out the receiving level measurement processing for all of these increased number of spread codes. Since this receiving level measurement processing requires a time as much as multiple times of the spread code period, when a number of spread codes is increased, there arises a problem that an activation time of the mobile station becomes even longer.